1. Field of the Invention
The present invention relates to papermakers fabrics, in general, and to dryer fabrics incorporating meltable yarns, in particular.
2. Background of the Prior Art
A conventional dryer felt or fabric consists of an endless conveyor belt, typically made from a two, three or more plane fabric, wherein the various planes are defined by different groups of cross-machine direction yarns. The planes, plies, or layers, as they are various called, are united by a plurality of machine direction yarns.
The yarns used to weave the most up-to-date dryer fabrics are generally made from synthetic monofilaments or synthetic multifilaments, from such materials as polyester or polyamide. Dryer felts made exclusively from monofilament yarns have certain drawbacks. Because the monofilament yarns are relatively stiff, they are not easily bent around each other during the weaving process. Thus, the fabric which results has a relative open structure. There are a number of positions on the papermaking machine that do not run or cannot run effectively when employing a very open fabric because of numerous problems with the paper sheet, such as thread-up, blowing, and flutter which causes sheet breaks.
A number of attempts to reduce the openness or permeability of dryer fabrics made predominantly of monofilaments have been tried. The major approach has been to use a bulky spun yarn as a stuffer pick in the middle of the weave pattern. These stuffer picks are, in effect, surrounded by the original monofilament cross-machine direction picks, which are positioned in both the face and back surfaces of the fabric. This approach has been successful in reducing permeability, but has added little or nothing to the stability of the fabric. It has also created the disadvantage that the spun stuffer pick is prone to collect dirt. Also, the stuffer picks have a tendency to retain and carry moisture, a condition which is undesirable. Therefore, a low permeability monofilament fabric produced with a spun stuffer pick runs wetter and gets dirty relatively quickly, compared to a high permeability all monofilament product.
A second approach has been to modify the weave structure in such a way that the top or face cross-machine direction picks are offset in relation to the bottom or back cross-machine direction picks. Although this approach has produced relatively low permeability in an all monofilament fabric, there is no easy way to change permeability. The weave design does not permit the use of stuffer picks. Therefore, the only changes are reducing the pick level from maximum (the number of weft or cross-machine direction yarns per inch), which, in turn, reduces the stability, or changing the number of warp or machine direction ends per inch, which necessitates redrawing the loom. Changes in yarn diameter are, of course, possible, but such changes can only be made within the limitations of the loom.
Yet another example of a way to obtain low permeability in a dryer felt is the incorporation of warp yarns of rectangular cross-section into a weave pattern that does not include provision for stuffer picks. In such a weave pattern, the warp yarn typically floats on the paper-receiving surface of the fabric over a number of weft picks. The longer the float, i.e., the more picks the warp yarn crosses before weaving back into the fabric, the less stable the fabric becomes. In this way, there is a tradeoff between permeability and fabric stability.
There is thus a need for papermakers fabric which may be easily and economically produced to provide a wide permeability range, which is stable and also dirt resistant, and which exhibits reduced moisture carrying properties. The present invention is directed toward filling that need.